An ultrasonic wave generator utilizing mechanical vibrations of piezoelectric effect is conventionally known widely. In the ultrasonic wave generator utilizing mechanical vibrations, electrodes are provided on both sides of a crystal of piezoelectric material such as barium titanate, and electric energy is supplied between both electrodes so that mechanical vibrations are generated. Thus, ultrasonic waves are generated with vibrating medium such as air. The ultrasonic wave generator utilizing mechanical vibrations, however, has inherent resonance frequency, so that frequency bandwidth of ultrasonic waves generated thereby is narrower. In addition, the ultrasonic wave generator is easily affected by outside oscillation or drift of outside pressure.
On the other hand, for example, as described in Japanese Laid-Open Patent Publication No. 11-300274 or Japanese Laid-Open Patent Publication No. 2002-186097, a pressure wave generator utilizing a method for forming coarseness and minuteness of air with thermal induction by which heat is given to medium is suggested as a device generating ultrasonic waves without being accompanied with mechanical vibrations.
As shown in FIGS. 35 and 36B, the pressure wave generator utilizing thermal induction comprises a semiconductor substrate 1 of a single crystalline silicon substrate, a thermal insulation layer 2 formed in the semiconductor substrate 1 inwardly from a face to a predetermined depth in thickness direction of the semiconductor substrate 1, and a heating conductor 3 of metallic thin film (for example, Al thin film) formed on the thermal insulation layer 2. The thermal insulation layer 2 is formed of porous silicon layer, and has a heat conductivity and volume heat capacity, which are much smaller than those of the semiconductor substrate 1.
When alternating current is supplied to the heating conductor 3 from an AC power source Vs, the heating conductor 3 runs hot, and temperature (or calorific value) of the heating conductor 3 varies corresponding to frequency of the alternating current. On the other hand, since the thermal insulation layer 2 is formed just below the heating conductor 3 and the heating conductor 3 is thermally insulated from semiconductor substrate 1, heat exchange effectively occurs between the heating conductor 3 and air in the vicinity. Then, expansion and contraction of air is repeated corresponding to variation of temperature (or variation of calorific value) of the heating conductor 3. Consequently, pressure waves such as ultrasonic waves are generated (an arrow of direction shows traveling direction of pressure waves in FIG. 35).
Such a pressure wave generator utilizing thermal induction can widely vary frequency of ultrasonic waves by varying frequency of alternating voltage (drive voltage) applied to the heating conductor 3. Therefore, it can be used as an ultrasonic wave source or a sound source of a speaker.
According to the above-mentioned Japanese Laid-Open Patent Publication No. 11-300274, it is desirable to make heat conductivity and volume heat capacity of the thermal insulation layer 2 smaller than those of the semiconductor substrate 1. In addition, it is preferable that a product of the heat conductivity and the volume heat capacity of the thermal insulation layer 2 is much smaller than a product of the heat conductivity and the volume heat capacity of the semiconductor substrate 1. For example, when the semiconductor substrate 1 is formed of a single crystalline silicon substrate and the thermal insulation layer 2 is formed of porous silicon layer, the product of the heat conductivity and the volume heat capacity of the thermal insulation layer 2 becomes about 1/400 of the product of the heat conductivity and the volume heat capacity of the semiconductor substrate 1.
For forming the thermal insulation layer 2 of porous silicon layer in a side of a first surface of the semiconductor substrate 1 of a single crystalline silicon substrate, as shown in FIGS. 37A and 37B, a masking layer having an opening at a portion where the thermal insulation layer 2 is to be formed is formed on a face of the semiconductor substrate 1. Then, an energizing electrode 4 is entirely formed on another face of the semiconductor substrate 1 is used as an anode, and an electric current is supplied between a cathode that is disposed to face the face of the semiconductor substrate 1 in an electrolyte so as to perform anodization processing.